How to extend a class within a Ruby module

Question

I've been using OilyPNG (a set of C extensions for working with PNG's) and I'd like to start adding some of my own methods. As such, I coded up a simple little C extension that creates the class Hola and includes the methods Hola.bonjour (which responds with a simple "hello") and Hola.wazzup(input) which spits the input right back.

OilyPNG is a module that contains a single class :Canvas with a bunch of methods :decode_png_image_pass, :from_data_url, :from_rgb_stream, etc.). I'd like to add my methods (bonjour and wazzup) to the Canvas so I can start using them. How do I do this?

It seems like this is a very basic Ruby question, but I can't seem to find it addressed anywhere. I've tried (among MANY other things) the following:

module ChunkyPNG
  class Canvas
    include Hola
  end
end

...but it returns the following:

Error: #TypeError: wrong argument type Class (expected Module)

All I want to do is something simple like creating a new Canvas object, then running my method, i.e. OilyPNG::Canvas.new.bonjour and I'm totally stuck.

UPDATE: The C code used to generate the extension

#include <ruby.h>

/* our new native method; it just returns the string "bonjour!" */
static VALUE hola_bonjour(VALUE self) {
    return rb_str_new2("bonjour, Zack!");
}

void Init_hola(void) {
    VALUE klass = rb_define_module("Hola"); /*DEFINED AS A MODULE*/

    rb_define_singleton_method(klass, "bonjour", hola_bonjour, 0);

}

UPDATE 2:

Finally got it working - using rb_define module in the C code, followed by defining the methods as rb_define_method (instead of as singletons). Then build the gem, install it, then require the .so file, then open up OilyPNG to include the Hola module, as follows:

module OilyPNG
  class Canvas < ChunkyPNG::Canvas
    def your_method
      puts "hello"
    end
      include Hola
  end
end

Then load a png file as a test object:

test = OilyPNG::Canvas.from_file("C:/code/testfiles/orange.png"); nil

(the nil is to prevent Sketchup from crashing when it tries to send the PNG object to the command line), and then testing the output of:

test.bonjour

And it works!

Mitch VanDuyn · Accepted Answer · 2016-02-20 02:35:55Z

1

module OilyPNG
  class Canvas < ChunkyPNG::Canvas # You must match the superclass when opening class and adding more definitions
    def your_method
      puts "hello"
    end
    include Hola # this will "insert" the Hola module at this point in the code... not sure what Hola contains so, not sure if that works or not.
  end
end

Looking at the source for OilyPNG the above should work...

Not sure why you got that error. Given this works here is why: You have to match the classes, super class when reopening it.

Based on this article: http://www.rubyinside.com/how-to-create-a-ruby-extension-in-c-in-under-5-minutes-100.html by the great Peter Cooper, I believe you should be using rb_define_method, not rb_define_singleton_method

If you are still having trouble, start with peter's example, then adapt it to your needs.

edited Feb 20, 2016 at 2:35

answered Feb 19, 2016 at 23:35

Mitch VanDuyn

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5 Comments

asetniop Over a year ago

I've converted Hola to a module (see above) and tried out your code; Ruby likes it just fine. And the your_method bit works just fine, but trying out the method from Hola it just kicks back an "undefined method" error. And on its own, Hola.bonjour still works. So bizarre.

Mitch VanDuyn Over a year ago

can you post a small sample of what Hola looks like. It seems that Hola is a module, but yet you say "As such, I coded up a simple little C extension that creates the class Hola" so perhaps somewhere in this is the problem

asetniop Over a year ago

Sure - I've added the code above (simplified a bit; I pulled out the wazzup method).

asetniop Over a year ago

Thanks - I actually can't even get Peter's code to work in irb (gets stuck on the require step). I've tried out the rb_define_method path and that's no go either; the method doesn't show up at all anywhere. If nothing else, I've learned a bit here, but this has been incredibly frustrating. I need to set it aside or I'll go insane.

asetniop Over a year ago

Well, I'm glad I didn't set it aside. Finally got it working, and your stuff was a huge help. I'm very grateful!

mu is too short · Accepted Answer · 2016-02-19 23:44:30Z

1

You say that you

[...] coded up a simple little C extension that creates the class "Hola"

so Hola is a class. But include expects a module, not a class. You have to rebuild your Hola as a module if you want to include Hola to patch ChunkyPNG::Canvas.

Consider this example:

class C
end
class D
  include C
end

That will give you exactly the same TypeError that you're seeing. However, if you say:

module M
end
class E
  include M
end

then it will work just fine.

answered Feb 19, 2016 at 23:44

mu is too short

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6 Comments

asetniop Over a year ago

Thanks - I've done that (using VALUE klass = rb_define_module("Hola"); in the C code) and the error goes away, but unfortunately the new methods don't show up (if I try to use them, it kicks back an "undefined method" error and if I list the methods they aren't there).

mu is too short Over a year ago

So how are you defining your methods?

asetniop Over a year ago

Do you mean inside the ruby environment? Just by requiring the .so file from the installed gem.

mu is too short Over a year ago

No, how would I go about reproducing your problem?

asetniop Over a year ago

It might be tough; it's taking place within the Ruby interface of Sketchup (plus there are a couple of additional layers; I'm packaging things as a gem and compiling that way cause I need it to run on Windows in Ruby 2.0.0 to work with Sketchup). I'm worried that my problems are a result of that stuff; everything I've seen here WORKS both on a conceptual level and in fundamental ways in the interface, but just no love for my C extensions. Probably my best bet is to pore through the OilyPNG source and see what's being done there that I'm missing. I really do appreciate the help, though.

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